Fifth disease is caused by an infection of human parvovirus B19. There is another parvovirus found in animals, but it is different than the human form, and cannot be transmitted to humans and vice versa.
Fifth disease often masquerades as a harmless rash but can develop into something much more serious if not treated. Forty to 60 percent of children between the ages of 5 and 15 have already caught the highly contagious virus, and outbreaks tend to happen in spring and winter.
The infection usually starts with a red rash on the face, which then spreads to the torso, arms and legs. Fifth disease is contagious, but usually before the rash appears. Once a child is infected, he or she develops immunity and usually does not run the risk of re-infection.